Transmission is a free, lightweight BitTorrent client. It features a simple, intuitive interface on top on an efficient, cross-platform back-end.

Transmission sets initial preferences so things "Just Work", while advanced features like watch directories, bad peer blocking, and the web interface can be configured with just a few clicks. Macworld put it this way: "It's fast, it's extremely lightweight, and â” even though it's available for a variety of platforms â” it behaves just as you'd expect a Mac program to."

Its code is freely available online and is licensed under either the GNU Public License v2 or the MIT License. The development team welcomes anyone who is interested in contributing code, documentation, translations, or other help.

I was an happy user of Transmission, and hoped for a while for the 0.8 version.
But I'm sorry to say, it's a failure : it always bug and crash, and I must restart my mac to use it after having done any little change.
Perhaps they will soon offer a 0.8.1 with bug fix ?

@ian_groh : It depends. Azureus has more features, but requires more resources. Eg Azureus supports transfer encryption. This is a feature that requires lots more CPU resources, but can result in faster transfers.
I use Transmission when I do work at the same time and I use Azureus when something downloads while I sleep.

Exactly the lightweight solution that prevents a torrent download from giving me the Azureus nightmare.

I think it should really create the files at their full size when downloads start, though. I saw some files being destroyed when a disk ran full because of that when I tried an eaerlier version. So: nice if you are sure your hard drive won't run full.

Nice, lightweight. But it froze my internet connection. Upload and download eventually would go down to 0kb/s and would even stop Firefox downloads. Restarting OS X/Transmission would only allow me to go slightly longer. Moved to BitRocket. Seems to have all that Transmission has, but more without being a heavyweight.

Comparing this with Azureus is like trying to match a Mac with a PC wreck anno 1997.

Azureus - which disappointingly seems to be the 'de facto' standard client - is so accumulated of bloat and a mess to configure (I hear it doesn't even ship with the proper settings pre-configured for optimal efficiency).

Transmission is a Mac app.

It would be a shame not to see it being considered legitimate on trackers in general, if not to say, a horrible tragedy. But now, seeing that they've changed a whole lot in the app, and hopefully conformed to most of the general praxis, there seems to be a chance for a very welcome revival! Granted, I know that at least one of the vital trackers out there has changed their policy towards allowing Transmission once again.